Manufacture of calcium sulphate anhydrite from gypsum, calcium carbonate and sulfuric acid



assignors'to National Lead-Company, New York, N. Y., a corporationofNew'dersey No Drawing; Application February 10, 1954 Serial N0.409,506

" Claims: (Cl. 23-122) The present iuven'tion-= relates" to a method forthe preparation of anhydrite calcium/ sulfate possessi'ngex cellentpigment properties and especially adapted for" the manufacture ofcomposite titanium-"-dioxide-calcium sulfate pigments.

The present application is a continuation-in-part of co-pending;application, Serial Number 260,328,.filedl December 6, 1951, nowabandoned...

Prior to the present invention, calcium sulfate anhyatent 25, other:objectswf therpresent invention :will become" apparentfrem the ensuingdescription;

Int itsabroad'est aspect the present invention Centaur-- plates mixing:together finely-divided". calcium carbonate sulfate: anhydrite' fromthemotherliquor;

The preferred fform t of IcalciumI-carbonate for the prac tice of theipresent. inventioniis -finely divided limestone which is ground to lessthan 325 m'esh (44 microns, U. S.

standard). The gypsum may be any available form'of.

this material, ei ga natural gypsum ground: to about the sam'e particlesize-as the limestone. However, a" pre ferred form of-igypsunr is tlratobtained according to the teaching of co-pending application, SerialNumber 260,328. According to that teaching, finely-divided limestoneground to below 325 mesh is reacted with a sulfuric acid liquor having aconcentration of H 80 below about' 40'3percent and preferably within therange of 4 percent to percent. A type of sulfuric acid liquor drite hasbeen. artificially prepared in several ways. Ac- 25 especially suitedfor the Preparation of the gypsum is cording to one method, as describedby, Washburn et al, in U. S. patents No. 1,906,730 andv No. 2,222,385,the anhydrite-is formed directly by adding an alkalinereactingcompound-of calcium, e. g. hydrated lime, to

hydrolysis mother liquor obtained after the hydrolytic precipitation ofthe titanium values from titanium sulfate solutions in the manufactureof titanium dioxide pigments.-:. Such hydrolysis liquors generallycontain from strong sulfuric acid under such conditions of concen-- 3abbutlo Percent to about 25 Percent H2504 The quan' tration,temperature, etc. as to maintain dehydrating. conditions throughout thereaction whereby anyhydriteis formeddirectly without passingthrough anhydrated phase. According-1o another method asdescribed-by' tity of.sulfuric acid employed is somewhat in excess of. the stoichiometricquantity required to react with the calcium carbonate, neutralization ofthe acid being carried-out to from about 75 percent to about 95 percentSulli i U s patent 2 5 339 sulfiu-ic acid'flis', 'of'comjpletion. It isimportant when utilizing waste sulneutralized with an' alkaline calciumcompound to obtain a product which contains substantial amounts ofgypsum, CaSO -2H 0. Thismaterial is converted"to' anhy-' drite by cokingor digesting .in. anaqueous sulfurieacid furic acid liquors to maintainsome acid in the final mother liquor'in order to prevent precipitationof impurities such as iron, vanadium, chromium and manganese. When puredilute sulfuric acid is used with relatively pure l i According) mmas-describ-edv-by 40 limestone, the reaction, that is to say, theneutralization Sullivan in :U. Si Patent No. 2,197,953, strong sulfuricacid is partially'neutralized with an alkaline calcium compound formingparticles of anhydrite suspended in asulfuric acid medium. Sullivanrefers to this as a 'seed suspension and to it is added finely-dividedgypsum and the mixture is" then heated to convert the gypsum toanhydrite;

All of the foregoing methods have'certaineconomic disadvantages: Themethod of Washburn et al., because of the necessity of maintainingdehydrating conditions, produces a waste mother liquor containingrelatively large amountsof'valuable' acid. Sullivans first methodrequires a rather prolonged, expensive heating treatment to effectconversion'of gypsum to anhydrite. His second method requires careful"control of'th'econditions for forming the seedsuspension and for theconversion of the added gypsum to"anh'ydrite. By means of the presentinvention, these disadavntagesare overcome bymeans of a simple,inexpensiveprocessv which produces'high quality anhydrite; Accordingt'oan economically-important embodiment' of the invention, wastesulfuric acid liquors are employed-as a .source of sulfate values fromwhich the anhydrite is-produced..

The principal object of the invention is, therefore,

the economical production of high quality calcium sulfate anhydrite.Another object of'the invention is the preparation of calcium sulfateanhydrite useful in the preparationof composite: titanium"dioxide-calcium sulfate pig merits; A third important object is aprocess for the utili zation' of wast'eisulfuric aci'c l liquors in-theprep'aration of high quality calcium sulfate anhydrite. These and ofthe sulfuric acid, may go substantially to completion.

According to the teachings of the said Serial Number 260,328,1the groundlimestone in the form of an aqueous slurry and the sulfuric acid liquorare flowed separately "andsimultaneously into a suitable reaction vesselunder normal conditions' of temperature:andzrpressurei- Agita tionshould be.ernployed during; the-mixing; but the use of -mechanicalagitators maybe avoided by delivering, th'ei: limestone slurry onto. asplash: platersituatedi' at: tth'e. top 'iof the reaction vessel andthe'swaste'racidi in 'a' stream: thamisuangential to the circumferenceofuthe :vessel'. The: reaction is practically" instantaneous-'- and:substantially;

ire; the wash ing; may not be"necessary where theiwastc acid is very lowin impurities, but in general, it is'desira'ble' to wash thedewateredgypsum."

The gypsum so obtained is ready for mixing with'lime-= stone for thepreparation of 'anhydritefiaccording to'th'e presenfiinvention;

The relative proportions-of limestone-' to gypsum are notcritical, butmay vary: widely; Forthe m'anufactureof anhydrite to" be subsequentlyemployed in' titanium pigment; manufacture, equal parts-"of limestoneand gypsum ('orr the basis of Ca) have been found' to yield Iasati'sfactot-y 'm-ixturei At-thepresent time, sulfuric'acid is amaterial in short supply because of the shortage of Patented: Feb. 4, 8:

tween the sulfuric acid and the limestone in the slurry stantiallyhigher than this point because it has been found that highertemperatures tend to promote. the formation of anhydrite of largeparticle size, larger, in fact, than is desirable for anhydrite to beemployed in the manufacture of composite titanium pigments. It has beenfound, further, that particle size of the initially formed anhydrite,the first 15 percent or less, influences the particle'- size of thesubsequently formed anhydrite. It is important to insure that theparticle size of the initially formed favorably influences the formationof small, fine particles in anhydrite, because of its large particlesize, does not of pigmentary useful anhydrite; (3) it furnishes make-uprender the batch of anhydrite unsuitable for pigment use. anhydrite forcomposite titanium pigment manufacture, This is best accomplished bymaintaining the temperasince the Waste acid does not contain sufiicientsulfate ture of the sulfuric acid at the start of the treatment atcontent to produce sufiicien-t anhydrite to meet plant reabout 60 C. Asthe limestone-gypsum slurry is added quirements; (4) it avoids excessdilution resulting from 15 to the sulfuric acid, the temperature beginsto rise bedehydration of the gypsum; and (5) it forms a subcause of theexothermic nature of the reaction between stantially neutral slurrypermitting the use of iron and the limestone and acid. It then tends tofall, and heat is steel equipment. supplied to the batch and continueduntil completion of The mixing of the gypsum and limestone, preferablyas the addition. Table I shows the temperature gradients slurries, isaccomplished before treatment with strong sul- 20 for several differenttreatments in which the limestonefuric acid. The acid should be at least55 percent H 80 to-gypsum ratio differed widely.

TABLE I Percent 02280 from Gypu None so so 90 95 100 Percent CaSO; fromCaO 100 50 20 10 5 None (so Bamso. at so 0. at start) Time, Minutes:

60 e1 60 60 so as e5 e7 as 65.5 69 e9 e7 69 64.5 71 70 60 ea 62 73 72 e3e4 59 5..-- 74 73 62 so 6 75 Heat 60 58 53.

Applied 7 76 57 55 51.5 s 77 55 63.6 50 q 53 52 49 10 77 51. 5 so. 5 47.5 11 49 49 46.5 12 4s 48 Heat Applied 13 76 47 47 14 4s 46 4s 16 73 454a 17 'A ii ci A ii i A n d 6 Maximum temperature pp 8 pp pp 8 withapplied beat 126 122 123 104 102 101 Final temperature 101 102 103 101100. 5 101 Addition time, min 60 96 100 68 as 70 Seventy-eight percentsulfuric acid (60 B.) has been For further, more complete understandingof the presvery satisfactory for the practice of the invention. Theproportion of sulfuric acid to the calcium (Ca) content of thelimestone-gypsum mixture should be in sufficient excess over thatrequired to react with the limestone present in the mixture to provideat least 4 percent sulfuric acid in the final liquor, taking intoaccount the water content of the limestone-gypsum mixture, the water inthe strike acid, the water formed by reaction between the limestone andsulfuric acid, and the water of crystallization of the gypsum releasedduring conversion to anhydrite. Here also, the excess remaining in thefinal liquor should be as low as possible for reasons of economy. Agood, practical operating proportion is one that permits between about6.0 and 7.5 percent H 50 to re-..

main in the final liquor.

The treatment of the limestone-gypsum mixture with the sulfuric acid ispreferably effected by running a slurry of the mixture into the strongsulfuric acid slowly during a period of time which will vary dependingupon the amount of mixture to be added and the rate at which heat can besupplied to the strike. The time may vary with large batches betweenabout 60 and 100 minutes.

ent invention, the following examples are supplied solely forillustrative purposes:

Example I A concentrated slurry of ground limestone having an averageparticle size and a particle-size distribution as of that discussedabove was prepared. It contained about 40.8 tons of limestone(hereinafter the formula CaCO will be employed for limestone; H O forwater; H for'sulfuric acid, CaSO .2H O for gypsum and CaSO foranhydrite) and about 61.2 tons of H 0, a total of 102.0 tons. Ninety-twotons of this slurry was fed into a reaction vessel at the top by flowingit on a splash plate while simultaneously adding hydrolysis motherliquor to the vessel at the side tangentially'to the circumference. Thetotal amount of acid added was about 865.0 tons; containing about 48.0tons of percent H 80 which indicates that the liquor had an H 80 contentof about 5.5 percent H 50 During the addition, about 16.2 tons of carbondioxide gas was evolved. The neutralization was about 75 percentcomplete.

9.40.8 tons of product was obtained. Of this amount, about 63.3 tons wasCaSO .2I-I Of (containing about 50.0

'tons CaSO about 12.0, tons was H 80 and about 865.5 tonswas liquorcontaining the. impurities present in the hydrolysis waste acid. The940.8 tons of product was filtered and washed on rotary filters. Thefiltrate was discarded. The filter cake consisted of a thick, plasticmass of CaSO .2H O.

157.5 tons of the filter cake was repulped with an agitator and theslurry was pumped to a larger mixing vessel. There was then added tonsof the CaCO slurry containing about 4.0 tons CaCO and 6.0 tons H O whichhad been originally prepared. This mixture, consisting of about 62.6tons of CaSO .2H O (containing about 49.5 tons CaSO 4.0 tons of 02100and 101 tons of H 0, at 25 C. was allowed to flow slowly during a periodof about 100 minutes into 14.4 tons of 77.8 percent H SO at 60 C. whileagitating the mixture. Within the first two minutes of the addition, thetemperature rose slowly to about 67 C. and then began to decline. Afterabout minutes, heat was applied. At the end of the treatment, the finalliquor contained about 55.0 tons CaSO 7.3 tons H 80 and 118.0 tons H O.The recovered CaSO (anhydrite) was found to be excellently suited forthe manufacture of titanium dioxide, calcium sulfate composite pigmentsby the known, so-called coprecipitation method.

Example 11 To 50 tons of finely-divided natural gypsum-water slurrycontaining 50 percent solids, calculated as CaSO .2H O, was added 12tons of finely-ground limestone-water slurry containing 40 percentlimestone CaCO and 9.5 tons of Water yielding a mixed slurry in which 75percent of the total calcium, Ca, was replaced by natural gypsum. Afterthoroughly mixing this slurry, it was pumped into a reaction vesselcontaining 9.8 tons of 77.8 percent sulfuric acid at a temperature of 50C., agitating mechanically during the addition. The temperature roserapidly during the initial period of addition and thereafter receded. Asthe decreasing temperature approached 40 C., heat was applied throughsteam coils in the reaction vessel. After the completion of the additionof the mixed gypsum-limestone slurry, the resultant calcium sulfateproduct was recovered by filtration. It was found to be calcium sulfateanhydrite, CaSO easily adapted for the manufacture of composite titaniumdioxide calcium sulfate pigments.

It will be seen from the foregoing description of the present inventionand the examples thereof that it differs from the prior art in that theformation of anhydrite proceeds through the neutralization of thecalcium compound simultaneously with the conversion of gypsum toanhydrite. Heretofore anhydrite was formed either by directneutralization of sulfuric acid with the calcium compound, according toWashburn, or by means of a conversion of gypsum involving seeding ordigestion according to the methods of Sullivan. The acid employed in thepresent invention consists initially of substantially pure sulfuric acidsolution in which there are no seeds present and the invention does notinclude any period of digestion after completion of the addition of themixed gypsum limestone slurry to the sulfuric acid.

While this invention has been described and illustrated by the examplesshown, it is not intended to be strictly limited thereto, and othermodifications and variations may be employed Within the scope of thefollowing claims.

What is claimed is:

1. Method for the preparation of pigmentary calcium sulfate anhydritewhich comprises mixing together finelydivided calcium carbonate andfinely-divided gypsum with water to form a slurry, adding the resultantmixed gypsum-calcium carbonate slurry continuously to a solutionconsisting initially of at least percent H 80 at a temperature nothigher than about C., the amount of said solution being such as toprovide at least 4 percent sulfuric acid in the mother liquor after thereaction between the calcium carbonate and sulfuric acid is complete andmaintaining the reaction mixture under agitation and at an elevatedtemperature during the said addition.

2. Method according to claim 1 wherein the finelydividedcalcium-carbonate is ground limestone.

3. Method according to claim 1 wherein the finelydivided gypsum isground natural gypsum.

4. Method according to claim 1 wherein the finelydivided gypsum isobtained by reaction between an alkaline-reacting calcium compound anddilute sulfuric acid.

5. Method for the preparation of calcium sulfate anhydrite whichcomprises admixing finely-divided ground limestone with a sulfuric acidliquor containing from about 4 percent to about 25 percent H thequantity of sulfuric acid employed being in excess of the stoichiometricquantity required to react with the limestone, separating the resultantgypsum from the supernatant liquor and mixing said gypsum with groundfinely-divided limestone and water to form a slurry, adding the mixedgypsum-limestone slurry continuously to a solution consisting initiallyof at least 55 percent H 80 at a temperature of about 60 C., the amountof acid being such as to provide at least 4 percent sulfuric acid in themother liquor after the reaction with the sulfuric acid is complete,maintaining the reaction mixture under agitation and at an elevatedtemperature during said addition and when said addition is complete,separating the calcium sulfate so formed from the mother liquor.

References Cited in the file of this patent UNITED STATES PATENTS2,006,342 Booge July 2, 1935 2,021,910 Crundall Nov. 26, 1935 2,197,953Sullivan Apr. 23, 1940 2,213,907 Fleckenstein Sept. 3, 1940 2,222,385Washburn Nov. 19, 1940 2,313,844 Sullivan Mar. 16, 1943

5. METHOD FOR THE PREPARATION OF CALCIUM SULFATE ANHYDRITE WHICHCOMPRISES ADMIXING FINELY-DIVIDED GROUND LIMESTONE WITH A SULFURIC ACIDLIQUOR CONTAINING FROM ABOUT 4 PERCENT TO ABOUT 25 PERCENT H2SO4, THEQUANTITY OF SULFURIC ACID EMPLOYED BEING IN EXCESS OF THE STOICHIOMETRICQUANTITY REQUIRED TO REACT WITH THE LIMESTONE, SEPARATING THE RESULTANTGYPSUM FROM THE SYPERNATANT LIQUOR AND MIXING SAID GYPSUM WITH GROUNDFINELY-DIVIDED LIMESTONE AND WATER TO FORM A SLURRY, ADDING THE MIXEDGYPSUM-LIMESTONE SLURRY CONTINUOUSLY TO A SOLUTION CONSISTING INITIALLYOF AT LEAST 55 PERCENT H2SO4 AT A TEMPERATURE OF ABOUT 60*C., THE AMOUNTOF ACID BEING SUCH AS TO PROVIDE AT LEAST 4 PERCENT SULFURIC ACID IN THEMOTHER LIQUOR AFTER THE REACTION WITH THE SULFURIC ACID IS COMPLETE,MAINTAINING THE REACTION MIXTURE UNDER AGITATION AND AT AN ELEVATEDTEMPERATURE DURING SAID ADDITION AND WHEN SAID ADDITION IS COMPLETE,SEPARATING THE CALCIUM SULFATE SO FORMED FROM THE MOTHER LIQUOR.